Currently, nonwoven fabrics have been widely used in fields of daily commodities, sanitary materials and medical treatment materials. Moreover, in recent years, a research on, and a development of, an air-laid nonwoven fabric capable of being produced at a high speed, and having excellent bulkiness, air-permeability and liquid permeability have advanced. Many proposals have been made for such air-laid nonwoven fabrics in which staple fibers consisting of a synthetic resin, for example, a polyolefin resin and a polyester resin and having excellent handing property and mechanical properties are used (e.g., Patent Reference 1, and the like).
A high air opening property is important to the staple fibers for the air-laid nonwoven fabric, and thus the quality of the resultant air-laid nonwoven fabric is controlled by the opening property of the staple fibers. For example, according to the investigation performed by the inventors of the present invention, the staple fibers for an air-laid nonwoven fabric having sheath layers formed from a high density polyethylene and located at the peripheral surfaces of the staple fibers, for example, poly(ethylene terephthalate)/high density polyethylene core-in-sheath type composite fibers and polypropylene/high density polyethylene core-in-sheath type composite fibers as described in Patent Reference 2 exhibit an improved high air opening property. In the air laid web formed from the above-mentioned conjugate staple fibers, the contents of unopened fiber bundles, in each of which several tens of staple fibers are arranged in parallel with each other, pill-like defects formed by entanglement of staple fibers are low and thus the resultant nonwoven fabric has an improved web quality compared with conventional fabrics.
However, even when the staple fibers as disclosed in the Patent Reference 1, and the conjugate fibers as disclosed in Patent Reference 2, namely conjugate fibers having high density polyethylene sheath components, are employed, the resultant nonwoven fabric is unsatisfactory in quality, because the water content, the individual fiber thickness and crimping condition of the staple fibers or the conjugate staple fibers influences on generation of defects on the resultant nonwoven fabric, and the generation of the defects has not yet been sufficiently controlled.
Patent Reference 1: WO 97/48846
Patent Reference 2: Japanese Unexamined Patent Publication (Kokai) No. 11-81116